Nonnative Rainbow Trout Oncorhynchus Mykiss Research Articles

Evaluating the effects of selective passage of migratory Westslope Cutthroat Trout on nonnative admixture

AbstractObjectiveHybridization with nonnative Rainbow Trout Oncorhynchus mykiss is a primary threat to the persistence of Westslope Cutthroat Trout O. clarkii lewisi. Managers concerned with conserving migratory populations of Westslope Cutthroat Trout in the presence of Rainbow Trout often face the predicament of tolerating the spread of hybridization, intentionally isolating Westslope Cutthroat Trout populations, or suppressing Rainbow Trout source populations.MethodsSelective passage of migratory Westslope Cutthroat Trout above existing barriers is a management approach with the potential to limit hybridization, while minimizing the population impacts of barriers in populations with preexisting low levels of hybridization. We took a before‐after–control‐impact approach to evaluate a phenotype‐based selective passage management program for migratory Westslope Cutthroat Trout in the Jocko River, Montana.ResultOf the 364 genotyped individuals selectively passed upstream of a barrier, 82% had a proportion of Rainbow Trout admixture < 0.01 and 98% had < 0.10. Over 14 years (2005–2019), there was no significant increase in hybridization at sites (n = 12) upstream of the barrier, but metrics increased within some of our control (no barrier, n = 8) sites. This increase was greatest at a site just downstream from the barrier, suggesting that blocked fish might have spawned opportunistically in this adjacent tributary.ConclusionWe demonstrate that phenotypic traits were useful in implementing a selective Westslope Cutthroat Trout passage program that allowed managers to promote the migratory life history without increasing nonnative admixture in this watershed.

Predation By the Nonnative Rainbow Trout, Oncorhynchus Mykiss (Walbaum, 1792), on the Native Biota from Freshwater Environment of the Central Andes (Argentina)

Abstract This study aimed to examine the impact of the nonnative rainbow trout Oncorhynchus mykiss on the native biota that inhabits freshwater environments of the central Andes, Argentina. To assess the predation pressure on native Andean vertebrates and invertebrates, the stomach contents were taken from rainbow trout caught at three Andean sites. Results showed that the major prey items were the native torrent catfish Hatcheria macraei (100% in diet), Elmidae coleopterans (23%), and the native crab Aegla affinis (38%) in the rainbow trout stomachs collected from Cipolletti Dam (North zone), Yaucha River (Middle zone), and Vaina Stream (South zone), respectively. This preliminary study exhibits the predation pressure of the rainbow trout on native Andean species and suggests that policies are necessary to control the release of fry of this exotic salmonid in the Andes region.

Preliminary evaluation of non-native rainbow trout (Oncorhynchus mykiss) impact on the Cederberg ghost frog (Heleophryne depressa) in South Africa’s Cape Fold Ecoregion

We evaluated the impact of non-native rainbow trout Oncorhynchus mykiss on a population of endemic Cedarberg ghost frog Heleophryne depressa in the upper Krom River (Olifants-Doring River Catchment, Cape Fold Ecoregion). We compared H. depressa abundance (using kick-sampling and underwater video analysis) and environmental conditions between sites above and below a waterfall that marks the upper distribution limit of O. mykiss. Heleophryne depressa abundance was significantly greater above the waterfall than that below it, and, because there was no significant difference in measured environmental variables, O. mykiss presence is identified as the most likely explanation for the observed decrease in H. depressa abundance.

Mass immigration of juvenile fishes into a small, once-dried tributary demonstrates the importance of remnant tributaries as wintering habitats

The winter ecology of stream fishes is still poorly understood, especially in non-salmonid species. A small tributary (<4 m in width, <1 km in length) of the Otofuke River (central Hokkaido, northern Japan) dewatered in the summer of 2013, which allowed us to evaluate immigration after the summer draught. More than 10000 juveniles of Siberian stone loach Barbatula toni, Japanese dace Tribolodon spp., and non-native rainbow trout Oncorhynchus mykiss immigrated to the tributary in the four months following the summer dewatering. This suggests that even remnant tributaries may be critical wintering habitats for some juvenile fishes.

The density dependent interspecific competition between nonnative salmonids, rainbow trout and brown trout

Interspecific interactions play a key role in determining species assemblages when nonnative species are introduced. In the Chitose River system, Hokkaido, northern Japan, nonnative brown trout Salmo trutta may have replaced nonnative rainbow trout Oncorhynchus mykiss due to interspecific competition. The outcome of this competition is predicted to be influenced by the density of both species. Then, I conducted an enclosure experiment to test whether density-dependent interspecific competition between YOY (young-of-the-year) of these species may have explained the species replacement. Rainbow trout decreased their stomach contents in the high density treatment sympatry with brown trout, and growth rate of rainbow trout decreased in the high density treatments both allopatry and sympatry with brown trout. Stomach contents and growth of brown trout were not affected by competitor species (rainbow trout) or total fish density. These results imply that brown trout are competitively superior to rainbow trout. Therefore, there is the potential for the replacement of rainbow trout by brown trout through interspecific competition.

Predatory impact of non-native rainbow trout on endemic fish populations in headwater streams in the Cape Floristic Region of South Africa

Non-native rainbow trout Oncorhynchus mykiss have been widely introduced in the Cape Floristic Region (CFR) at the south-western tip of Africa and may pose a serious threat to endemic freshwater fishes in the region. Quantitative information about trout impacts in the CFR is scarce but is urgently needed to guide legislation and management efforts. We used a combination of comparative and experimental approaches to evaluate impacts of introduced trout on native fish populations in headwater streams draining the upper Breede River catchment in the CFR. Fish populations were surveyed, and 19 environmental variables measured, in 24 minimally-disturbed streams, half of which had been invaded by trout. The mean densities of native Breede River redfin Pseudobarbus burchelli, Cape kurper Sandelia capensis and Cape galaxias Galaxias zebratus, were 89–97 % lower in invaded streams than in streams without trout. Furthermore, while native fish were present at all 12 sites without trout, they were not recorded at seven of the 12 invaded sites. None of the measured environmental variables differed significantly between sites with and without trout, and distance-based linear models identified trout density as the best predictor of redfin and kurper density, while galaxias density was best predicted by other environmental variables (riparian vegetation, canopy cover, substrate length, site slope). Native fish ≤40 mm in length were largely absent from invaded streams, but generally abundant in streams without trout, and a field experiment confirmed that trout selectively consume small redfin. Taken together, these findings constitute evidence that trout have depleted the abundance of CFR-endemic fishes through size-selective predation. It is recommended that managers aim to prevent new trout introductions and consider eradicating trout populations where they pose a threat to highly threatened native species.

Quantifying the Effectiveness of Conservation Measures to Control the Spread of Anthropogenic Hybridization in Stream Salmonids: a Climate Adaptation Case Study

AbstractQuantifying the effectiveness of management actions to mitigate the effects of changing climatic conditions (i.e., climate adaptation) can be difficult, yet critical for conservation. We used population genetic data from 1984 to 2011 to assess the degree to which ambient climatic conditions and targeted suppression of sources of nonnative Rainbow Trout Oncorhynchus mykiss have influenced the spread of introgressive hybridization in native populations of Westslope Cutthroat Trout O. clarkii lewisi. We found rapid expansion in the spatial distribution and proportion of nonnative genetic admixture in hybridized populations from 1984 to 2004, but minimal change since 2004. The spread of hybridization was negatively correlated with the number of streamflow events in May that exceeded the 75th percentile of historic flows (r = −0.98) and positively correlated with August stream temperatures (r = 0.89). Concomitantly, suppression data showed a 60% decline in catch per unit effort for fish with a high proportion of Rainbow Trout admixture, rendering some uncertainty as to the relative strength of factors controlling the spread of hybridization. Our results illustrate the importance of initiating management actions to mitigate the potential effects of climate change, even where data describing the effectiveness of such actions are initially limited but the risks are severe.Received November 22, 2013; accepted March 1, 2014

The impact of non-native rainbow trout within Afro-montane streams in eastern Zimbabwe

Non-native trout species have been associated with many negative effects in receiving ecosystems. The first aim of this study was to determine the impact of non-native rainbow trout Oncorhynchus mykiss on distribution and abundance of native mountain catfish Amphilius uranoscopus within Afro-montane streams in Nyanga Mountains, eastern Zimbabwe. The second aim was to compare macro-invertebrate community responses to the presence of the trout and the catfish. We examined trout impact on catfish’s habitat associations, whereas macro-invertebrate composition was compared using open fish and fish exclosure experiments in habitats with and without trout. Trout influenced both the distribution and abundance of the catfish that occupied shallow reaches possibly to avoid predation from trout that occurred in the deeper habitats. Within trout invaded reaches, most macro-invertebrate taxa were more abundant in exclosure than open treatments. By contrast, within trout-free reaches, most macro-invertebrates either did not differ between treatments or were generally more abundant in open than exclosure treatments. This suggests that the macro-invertebrate communities responded differently within invaded and non-invaded reaches. By influencing distribution and abundance of native biota, non-native rainbow trout may have wider ecological effects, such as influencing trophic interrelationships within invaded habitats.

Effects of Fluctuating Flows and a Controlled Flood on Incubation Success and Early Survival Rates and Growth of Age‐0 Rainbow Trout in a Large Regulated River

AbstractHourly fluctuations in flow from Glen Canyon Dam were increased in an attempt to limit the population of nonnative rainbow trout Oncorhynchus mykiss in the Colorado River, Arizona, due to concerns about negative effects of nonnative trout on endangered native fishes. Controlled floods have also been conducted to enhance native fish habitat. We estimated that rainbow trout incubation mortality rates resulting from greater fluctuations in flow were 23–49% (2003 and 2004) compared with 5–11% under normal flow fluctuations (2006–2010). Effects of this mortality were apparent in redd excavations but were not seen in hatch date distributions or in the abundance of the age‐0 population. Multiple lines of evidence indicated that a controlled flood in March 2008, which was intended to enhance native fish habitat, resulted in a large increase in early survival rates of age‐0 rainbow trout. Age‐0 abundance in July 2008 was over fourfold higher than expected given the number of viable eggs that produced these fish. A hatch date analysis indicated that early survival rates were much higher for cohorts that hatched about 1 month after the controlled flood (∼April 15) relative to those that hatched before this date. The cohorts that were fertilized after the flood were not exposed to high flows and emerged into better‐quality habitat with elevated food availability. Interannual differences in age‐0 rainbow trout growth based on otolith microstructure supported this hypothesis. It is likely that strong compensation in survival rates shortly after emergence mitigated the impact of incubation losses caused by increases in flow fluctuations. Control of nonnative fish populations will be most effective when additional mortality is applied to older life stages after the majority of density‐dependent mortality has occurred. Our study highlights the need to rigorously assess instream flow decisions through the evaluation of population‐level responses.

Innocent until proven guilty? Stable coexistence of alien rainbow trout and native marble trout in a Slovenian stream

To understand the consequences of the invasion of the nonnative rainbow trout Oncorhynchus mykiss on the native marble trout Salmo marmoratus, we compared two distinct headwater sectors where marble trout occur in allopatry (MTa) or sympatry (MTs) with rainbow trout (RTs) in the Idrijca River (Slovenia). Using data from field surveys from 2002 to 2009, with biannual (June and September) sampling and tagging from June 2004 onwards, we analyzed body growth and survival probabilities of marble trout in each stream sector. Density of age-0 in September over the study period was greater for MTs than MTa and very similar between MTs and RTs, while density of trout ≥age-1 was similar for MTa and MTs and greater than density of RTs. Monthly apparent survival probabilities were slightly higher in MTa than in MTs, while RTs showed a lower survival than MTs. Mean weight of marble and rainbow trout aged 0+ in September was negatively related to cohort density for both marble and rainbow trout, but the relationship was not significantly different between MTs and MTa. No clear depression of body growth of sympatric marble trout between sampling intervals was observed. Despite a later emergence, mean weight of RTs cohorts at age 0+ in September was significantly higher than weight of both MTs and MTa. The establishment of a self-sustaining population of rainbow trout does not have a significant impact on body growth and survival probabilities of sympatric marble trout. The numerical dominance of rainbow trout in streams at lower altitudes seem to suggest that while the low summer flow pattern of Slovenian streams is favorable for rainbow trout invasion, the adaptation of marble trout to headwater environments may limit the invasion success of rainbow trout in headwaters.

Are environmental conditions in Finnish streams limiting to early life-history survival in the nonnative rainbow trout?

The nonnative rainbow trout Oncorhynchus mykiss has been an unsuccessful invader in North European streams, although it has been widely introduced. Here we studied whether early life history stages (egg incubation and hatching, first overwintering) act as filters for the establishment of hatchery rainbow trout. Survival of hatched alevins was approximately 80%, whereas only 47% of the embryos survived. However, the latter value was impacted by the high number of unfertilized eggs. Correlation coefficients with embryo survival rate and environmental variables (pH and temperature) were statistically insignificant. In the overwintering experiments, the survival of rainbow trout was 93%. The growth was generally slowed during the winter, but in the spring the growth of rainbow trout exceeded that of the native brown trout. Our data demonstrated that the survival and growth of rainbow trout during early life-history stages were relatively high and comparable to those of the native brown trout. Based on the variables considered in our study, our results suggest that environmental conditions during early life-history stages are not detrimental for rainbow trout in the study streams.

Comparative Effects of Rotenone and Antimycin on Macroinvertebrate Diversity in Two Streams in Great Basin National Park, Nevada

AbstractWe used rotenone and antimycin A to eradicate nonnative rainbow trout Oncorhynchus mykiss and brook trout Salvelinus fontinalis as part of a program to reintroduce Bonneville cutthroat trout O. clarkii utah in two streams in Great Basin National Park, Nevada. These treatments followed standard protocols for piscicide application and provided a unique opportunity to compare the effects of the two piscicides on macroinvertebrate diversity over a short spatial and temporal window. Antimycin A had minimal effects on macroinvertebrate diversity. Total species richness and that of Ephemeroptera, Plecoptera, and Trichoptera (EPT); total and EPT density; and Simpson index values were reduced significantly more with rotenone than with antimycin. The Hilsenhoff biotic index increased with rotenone but decreased with antimycin A. Rotenone also had greater effects on species composition. The proportions of EPT taxa, shredders, and filterers were less than expected when rotenone was used, whereas scraper proportions were greater than expected. The proportions with antimycin A did not differ from those expected. No taxa were lost from antimycin A–treated sites, but four taxa were still missing from rotenone‐treated sites 1 year posttreatment. Of these four taxa, however, two were collected 3 years posttreatment. We suggest that antimycin A be used rather than rotenone if minimizing the effects on macroinvertebrates is important. Recovery was relatively rapid with both piscicides, however, and if the short‐term impacts on macroinvertebrates are not a concern, the use of rotenone may be appropriate.

Hybridization and Cytonuclear Associations among Native Westslope Cutthroat Trout, Introduced Rainbow Trout, and Their Hybrids within the Stehekin River Drainage, North Cascades National Park

AbstractHistoric introductions of nonnative rainbow trout Oncorhynchus mykiss into the native habitats of cutthroat trout O. clarkii have impacted cutthroat trout populations through introgressive hybridization, creating challenges and concerns for cutthroat trout conservation. We examined the effects of rainbow trout introductions on the native westslope cutthroat trout O. c. lewisii within the Stehekin River drainage, North Cascades National Park, Washington, by analyzing 1,763 salmonid DNA samples from 18 locations with nine diagnostic nuclear DNA markers and one diagnostic mitochondrial DNA (mtDNA) marker. Pure westslope cutthroat trout populations only occurred above upstream migration barriers in the Stehekin River and Park Creek. Two categories of rainbow trout admixture were observed: (1) less than 10% within the Stehekin River drainage above the Bridge Creek confluence and the middle and upper Bridge Creek drainage and (2) greater than 30% within the Stehekin River below the Bridge Creek confluence and in lower Bridge Creek. Hybrid indices and multilocus genotypes revealed an absence of rainbow trout and reduced hybrid diversity within the Stehekin River above the Bridge Creek confluence relative to hybrid diversity in the Stehekin River below the confluence and within lower Bridge Creek. Cytonuclear disequilibrium statistics revealed assortative mating between westslope cutthroat and rainbow trout but not among hybrids within the same locations. This suggests that a randomly mating hybrid swarm does not currently exist. However, continual migration of parental genotypes into the study location could also create significant cytonuclear disequilibria. The Stehekin River represents a novel and unique example of a dynamic hybridization zone where the invasion of rainbow trout alleles into the Stehekin River westslope cutthroat trout population above the Bridge Creek confluence appears to be impeded, suggesting that divergent ecological or evolutionary mechanisms promote the population structure within the Stehekin River drainage, depending upon location.

Ebb and Flow of Encroachment by Nonnative Rainbow Trout in a Small Stream in the Southern Appalachian Mountains

Abstract Brook trout Salvelinus fontinalis is the native salmonid species of streams in the southern Appalachian Mountains. The present distribution of this species, once widespread from headwaters to lower reaches of large streams, is restricted to mostly headwater areas. Changes in the distribution of native brook trout in the presence of' nonnative rainbow trout Oncorhynchus mykiss have been documented in Great Smoky Mountains National Park. When rainbow trout were first found in a tributary (Rock Creek) in the park in 1979, a study was begun to assess changes through time in distribution and abundance of rainbow trout in Rock Creek and to compare the brook trout and rainbow trout associations in Rock Creek with associations found in other park streams. Abundance of brook trout was low in the downstream sections of Rock Creek in 1979–1993. Brook trout abundance was highest in the steep-gradient, pool-dominated headwater section which was only 2 km from the confluence of Rock Creek and Cosby Creek. Rain...